The present invention relates to data communication networks and particularly to equipment for testing the interface to the network.
Communication networks are used to interconnect a variety of data processing equipment. The network allows a given piece of equipment to communicate with different pieces of equipment at various times. In order for data processing equipment from a variety manufacturers to be able to communicate over the same network, the equipment must strictly adhere to a well defined protocol for the particular network.
The International Standards Organization (ISO) has developed an open system interconnect (OSI) model for a data communication network which consists of seven layers defining a hierarchy of network protocol functions. The first two layers of the model, designated the physical and link layers, define the physical characteristics of the network and how the access to the network medium is controlled. An important part of the model is the definition of the interface between the data processing equipment and the data network devices, such as modems. The definition is in terms of the logical, electrical and mechanical characteristics of this interface.
One type of network to which this model applies is a broad-band token passing bus system. The term broadband refers to the signals being sent and received on two separate frequency bands through a broad bandwidth medium. Each station on the network is coupled to the medium by a modem which transmits its data by modulating a signal at a first frequency. Head end equipment which controls the network, receives the transmitted signal, demodulates it and remodulates the recovered data on a signal at a second frequency. All of the network stations receive data by their modem demodulating this second frequency signal. Access to this network is controlled by passing the token from one station to the next. A given station can send messages on the network only when it holds the token and must periodically relinquish the token to the next station.
The data is sent over the network as a message frame consisting of a group of data symbols. Each message frame includes a header which is a series of data symbols that identify the start of the frame, a data field, and an end delimiter which signals the end of the frame. Although the contents of the data field vary from frame to frame, it must contain valid symbols, typically only those used to define one and zero data bits. This field cannot contain other symbols, such as pad-idle, and silence symbols, that are sent in the header and between message frames. Furthermore, the header and end delimiter must consist of a predefined symbol pattern in order for the frame to be properly transmitted over the network.
In order for a manufacturer to market interface equipment, such as modems, for a particular type of network, the equipment must be rigorously tested under a wide range of conditions and all conceivable operating modes to insure that it conforms to the protocol standards. The conventional technique for testing a network interface device previously involved attaching it to data terminal equipment which produced signals adhering to the protocol standard for the particular network. Message frames were then sent and received repeatedly via the interface device. The transmitted and received frames were compared to detect errors introduced by the device Since the transmission of message frames had to comply with the standard, the test transmission could not be continuous as the standard prevented any one station from holding the token for more than the prescribed time. By requiring the station testing the interface device to periodically relinquish the token even if it was the only station on the test network, the evaluation procedure became a prolonged process
When conventional data terminal equipment was used to test an interface device, other dictates of the communication protocol also had to be adhered to. For example, the terminal equipment could only produce valid message frames having headers, end delimiters and a total length which conformed to the standard protocol. Strict adherence to the defined standard often prevented testing the interface device under worst case conditions and examining how the device responded to different errors.